There are precious few preserved remains of either species A or B (especially B), so knowledge of their physiology is pieced together from various, sometimes conflicting sources.
Species A were moderately sized beings, between 1 - 3 meters in height, and no more than 2 meters in diameter.
Species B were smaller beings, no more than a meter wide, high, or thick.
The native habitats of these creatures are unknown. Given the nature of the planets they settled, it can be assumed that they were carbon-based lifeforms, and that at least of one of the two was an oxygen breather.
Beyond their technological advancement, little is known.
Excerpt from "A Brief History in Time and Space"
"While there is much contention about the nature of the predecessors of the Ancients, the Ancients themselves left enough rubble strewn around the galactic arm to convince even a fairly hardened skeptic of their having dwelled in these parts. The Ancients appear to have been made up of at least two major species groups, and interacted with at least three others, albeit it is not known whether these were client species, or contemporaries from another part of the galaxy. Their reign over this region lasted until about 1 to 2 million years ago, whereupon they rapidly ceased to be present. There is a wealth of evidence that severe infighting played some part in the destruction of the Ancients, but, assuming there were victors in such a conflict, little is known of what became of them.
The best source of such evidence, however limited, is the Uln homeworld. While they are quite sensitive about the subject, the widely held belief among the major races is that the Uln are the descendants of the Ancient's equivalents of lab monkeys. The Uln culture sprang up among the remains of a sprawling set of Ancient structures... if they hadn't been so ill prepared for the gifts they unintentionally received, they would have conquered the entire arm. Fortunately for the aspirations of dominance held by other species, the Uln were decidedly unprepared. Indeed, they spent so much time blowing each other up with weapons they didn't entirely control that it is a wonder that either they or the ruins on their planet still survive.
The ruins, however, did not escape unscathed from the genesis of the Uln culture. While assuredly the largest known source of information on the Ancients, the ruins deliver little coherent information about many key aspects of the Ancients' existences, largely due to vast portions of many buildings having been turned to dust."
To clarify, the statement about the Uln possibly having been able to conquer the arm is contingent upon them NOT having blown most of the ruins up, along with the devices they were using to blow each other up, and large portions of their own species. No superweapons as such are known to be currently possessed by the Uln. The Uln are in possession of one particularly impressive piece of Ancient technology (what is known as the Sul-Gatwa high castle), but it's potential is effectively limited to the defense of their homeworld. However, even as heavily damaged as the finds on the Uln world are, they remain the best source for archeological research, and research visas remain a large part of the Uln economy.
Why would this blasted planet be the best source of artifacts? Because, unlike most of the planets that seem to have been inhabited by the Ancients, it didn't have it's entire surface slagged, get broken into a debris field of billions of pieces of rock, or become pockmarked by craters implying assault equivalent to prolonged planetary bombardment by 500Km wide asteroids.
Despite this, the occaisional piece of debris is found in such places. However, the only officially reported finds of fully-functioning Ancient technology have been the nano-plague and various minor finds on the Uln homeworld.
The largest known piece of Ancient technology is the Sul-Gatwa high castle. Whether or not it is technically functional is a matter of some scholarly contention - the object is a slagged chunk of some small moon sized ship or station. No systems appear to be remotely functioning, however, what information has been declassified by the Uln Royal Ingatwa fleet and confirmed via espionage implies that the structure is so dense that it should have collapsed under its own gravity into a solid mass - however, as the material that the high castle is composed of defies the best efforts of science to explain or duplicate (it has been jokingly dubbed "unobtainium") it could just be some intrinsic property of the material and not evidence of functioning gravitics. While the high castle has none of it's original equipment and is, in essence a giant chuck of debris in orbit around the Uln homeworld, every indication is that it retains the potential to absorb absurd amounts of damage, and, as such, the centuries that the Uln have spent arming it with their own weapons have made it the most formidable planetary defense station known to exist. Its existence, the threat of the Uln destroying what remains of the Ancient artifacts, and general opinion that any of the major powers could pen the Uln into their home system if necessary are believed to be the primary factors responsible for the Uln remaining independant entities.
Small fragments of Ancient technology, even completely non-functional, fetch a fair price at any research facility or university planet. Functional pieces of Ancient technology, even if relatively useless, are of exceptional value, and it is not unheard of for persons to attempt to make a career out of artifact prospecting, subsisting on the rewards from finding small pieces of debris while waiting for the big catch of working Ancient tech. However, as time progresses, the easier pickings have already been scavenged, and exploration of progressively more hostile environments has become necessary to sustain the trickle of finds.